The oxidizing chemistry environment of BWR reactor water is a key factor in promoting intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel based alloys used to construct reactor coolant system piping and vessel internals. Intergranular stress corrosion cracking in BWRs is typically mitigated during power operation with hydrogen water chemistry (HWC) or noble metal chemical addition with hydrogen water chemistry (NMCA+HWC).
However, these methods are only completely effective when the reactor is at power. Hydrogen injection is not placed into service until the reactor is at operating temperature and at a power greater than about 5% to 30%, depending on HWC system design. Consequently, the reactor water, which initially contains high dissolved oxygen levels from exposure to atmospheric air during cold shutdown, is oxidizing during heatup and low power operation. As a result, the highest crack growth rates currently occurs during start up, after refueling outages, before HWC becomes effective. Further, electrochemical corrosion potential (ECP) is very high.
Data indicates that IGSCC rates are higher at intermediate temperatures during plant startup and shutdown processes than at operating temperature. As a result, cracking can initiate and crack growth can occur during the plant shutdown process and startup from refueling or mid-cycle outages, when hydrogen injection is not in service. For units with NMCA+HWC, crack growth during the startup and shutdown processes may result in crack flanking, in which existing cracks can continue to grow even after hydrogen injection is on.